The present disclosure relates to plasma processing methods using high-frequency electric discharge, and more particularly, to methods of plasma doping of a solid surface portion etc. with an impurity.
In recent years, as the integration density, performance, and speed of semiconductor integrated circuit devices have been increased, there has been a demand for a technique for lower-energy, higher-density and more conformal doping in order to achieve shallower junction and three-dimensional structure for transistor scaling. To achieve this, plasma doping has been studied as an alternative to conventional ion implantation.
In general, plasma doping can supply high current using low energy compared to ion implantation. The doping characteristics of plasma doping, however, depend significantly on conditions for a plasma, and therefore, plasma doping has a problem with injection efficiency, uniformity, controllability, etc. Therefore, various plasma generation techniques have been studied, and various modifications have been made to each plasma generation technique. For example, Japanese Translation of PCT International Application No. 2008-523625 describes a modified version of DC pulsed discharge. Japanese Patent Publication H11-329271 describes a modified version of high-frequency discharge. In these modified versions of the plasma generation techniques mainly try to improve the efficiency of ionization of a dopant species to be supplied. However, when the dopant is mainly supplied in the form of ions, there is unavoidably a problem with uniformity, stability, etc. which are limited by the plasma conditions as described above. In particular, instability during plasma ignition is unavoidable, directly leading to variations in the degree of doping. The directivity of ions as a dopant species, in principle, makes it nearly impossible to dope a side surface of a three-dimensional structure, such as a semiconductor region in the shape of a fin (hereinafter referred to as a fin-shaped semiconductor region) shown in FIG. 8.
In contrast to this, Japanese Patent Publication No. 2004-179592 describes a doping technique which provides good uniformity and time controllability by using a plasma containing a doping gas (e.g., B2H6) diluted to the extent possible with a noble gas (e.g., He). In the plasma doping technique of Japanese Patent Publication No. 2004-179592, the dilution ratio of the doping gas is set to be considerably low (less than 0.05%). It is presumed that most ions contained in the plasma are noble gas ions (He+ etc.), and the dopant species is supplied in the form of a neutral radical mainly containing boron (B) etc. to a surface of a material to be doped, although the underlying mechanism has not been elucidated. As shown in FIG. 8, unlike ions, the neutral radical does not have directivity, i.e., has isotropy. Therefore, in principle, the dopant can be supplied and attached to a three-dimensional structure uniformly (conformally), i.e., to vertical (side) surfaces as well as horizontal (top or bottom) surfaces.